How to get an Object's Key by Value in TypeScript

# Get an Object's Key by Value in TypeScript

To get an object's key by value in TypeScript:

Use the Object.keys() method to get an array of the object's keys.
Type the array to be an array of the object's keys.
Use the find() method to get the key by its value.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

// ✅ Using Object.keys()
const result1 = (Object.keys(obj) as (keyof typeof obj)[]).find((key) => {
  return obj[key] === 'accounting';
});

console.log(result1); // 👉️ "department"

get object key by value in typescript

The code for this article is available on GitHub

If you need to get an object's value by key, use bracket or dot notation instead.

index.ts

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// ✅ bracket notation
console.log(obj['department']); // accounting

// ✅ dot notation
console.log(obj.name); // Bobby Hadz
console.log(obj.country); // Chile

If the name of the key is stored in a variable, use bracket notation.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

const key = 'department';

// ✅ with key stored in a variable
console.log(obj[key]); // accounting

We used the Object.keys() method to get an object's key by its value.

The method returns an array of the object's keys.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

// 👇️ const r: string[]
const r = Object.keys(obj);
console.log(r); // 👉️ ['name', 'department', 'country']

However, note that TypeScript types the return value of the Object.keys() method as string[].

All of the object's keys are strings, but not all strings are keys in the object, so we wouldn't be able to access a value by key directly.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

// 👇️ const r: string[]
const r = Object.keys(obj).find((key) => {
  // ⛔️ Error:  No index signature with a parameter of
  // type 'string' was found on type
  // '{ name: string; department: string; country: string; }'.
  return obj[key] === 'accounting';
});

TypeScript is telling us that we can't index the object with any string key, it has to be name, department or country.

I've also written an article on how to create a type from an object's keys or values.

# Using a type assertion to resolve the issue

This is why we used a type assertion to type the return value of the Object.keys() method.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

// now the `key` parameter is typed as
// 👇️ (parameter) key: "name" | "department" | "country"
const result1 = (Object.keys(obj) as (keyof typeof obj)[]).find((key) => {
  return obj[key] === 'accounting';
});

console.log(result1); // 👉️ "department"

using type assertion to resolve the issue

The code for this article is available on GitHub

Now the key parameter in the find method is a union type of the objects keys, so everything works as expected.

The function we passed to the Array.find method gets called with each element in the array of keys until it returns a truthy value or iterates over the entire array.

If the condition is met, the find() method returns the corresponding array element and short-circuits.

If the condition is never satisfied, find() returns undefined.

Want to learn more about working with objects in TypeScript? Check out these resources: Dynamically access an Object's Property in TypeScript,How to Add a property to an Object in TypeScript,How to initialize a typed Empty Object in TypeScript.

# Using the `Object.entries()` method to get an object's key by value

You can also use the Object.entries() method to get an object's key by its value.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

let result2 = '';

Object.entries(obj).find(([key, value]) => {
  if (value === 'accounting') {
    result2 = key;
    return true;
  }

  return false;
});

console.log(result2); // 👉️ "department"

using object entries method to get an object key by value

The code for this article is available on GitHub

Notice that we don't have to use type assertions when using this approach.

The Object.entries() method returns an array of key-value pairs.

index.ts

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const obj = {
  name: 'Bobby Hadz',
  department: 'accounting',
  country: 'Chile',
};

// 👇️ const r: [string, string][]
const r = Object.entries(obj);

// 👇️ [['name', 'Bobby Hadz'], ['department', 'accounting'], ['country', 'Chile']]
console.log(r);

This is useful because we don't have to use a type assertion as we get an array of 2 elements - the key and value.

The find() method returns the array element for which the condition is met or an undefined value and we only want the key, so it's simpler to:

Initialize a variable outside of the method
Set its value once the condition is met
Short-circuit

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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How to get an Object's Key by Value in TypeScript

# Get an Object's Key by Value in TypeScript

# Using a type assertion to resolve the issue

# Using the Object.entries() method to get an object's key by value

# Additional Resources

Borislav Hadzhiev

# Using the `Object.entries()` method to get an object's key by value